Steam cracking has long been used to crack various hydrocarbon feedstocks into olefins, preferably light olefins such as ethylene, propylene, and butenes. Conventional steam cracking utilizes a furnace which has two main sections: a convection section and a radiant section. The hydrocarbon feedstock typically enters the convection section of the furnace as a liquid or gas wherein it is typically heated and vaporized by indirect contact with hot flue gas from the radiant section and by direct contact with steam. The vaporized feedstock and steam mixture is then introduced into the radiant section where the cracking takes place.
Conventional steam cracking systems have been effective for cracking high-quality feedstock which contains a large fraction of light volatile hydrocarbons, such as naphtha. However, steam cracking economics sometimes favor cracking lower cost feedstocks containing resids such as, atmospheric resid and crude oil. Crude oil and atmospheric resid often contain high molecular weight, non-volatile components with boiling points in excess of 590° C. (1100° F.). Cracking heavier feeds produces large amounts of tar. There are other feeds, such as gas-oils and vacuum gas-oils, that produce large amounts of tar and are also problematic for conventional steam cracking systems.
In conventional chemical manufacturing processes, steam cracker tar is typically an undesired side product. When large volumes of low value steam cracker tar are produced by the refinery, the refiner is placed in the position of blending the tar into heavy fuels or other low value products. Alternatively, steam cracker tar can be used as a fuel within the refinery; however, its physical and chemical properties make it extremely difficult to burn cleanly and efficiently.
Burners used in large industrial furnaces typically use either liquid or gaseous fuel. Liquid fuel burners typically mix the fuel with steam prior to combustion to atomize the fuel to enable more complete combustion, and mix combustion air with the fuel at the zone of combustion.
Gas fired burners can be classified as either premix or raw gas, depending on the method used to combine the air and fuel. They also differ in configuration and the type of burner tip used.
Raw gas burners inject fuel directly into the air stream, such that the mixing of fuel and air occurs simultaneously with combustion. Since airflow does not change appreciably with fuel flow, the air register settings of natural draft burners must be changed after firing rate changes. Therefore, frequent adjustment may be necessary, as explained in detail in U.S. Pat. No. 4,257,763, which patent is incorporated herein by reference. In addition, many raw gas burners produce luminous flames.
Premix burners mix the fuel with some or all of the combustion air prior to combustion. Since premixing is accomplished by using the energy present in the fuel stream, airflow is largely proportional to fuel flow. As a result, therefore, less frequent adjustment is required. Premixing the fuel and air also facilitates the achievement of the desired flame characteristics. Due to these properties, premix burners are often compatible with various steam cracking furnace configurations.
Floor-fired premix burners are used in many steam crackers and steam reformers primarily because of their ability to produce a relatively uniform heat distribution profile in the tall radiant sections of these furnaces. Flames are non-luminous, permitting tube metal temperatures to be readily monitored. As such, the premix burner is the burner of choice for such furnaces. Premix burners can also be designed for special heat distribution profiles or flame shapes required in other types of furnaces.
The majority of recent burner designs for gas-fired industrial furnaces are based on the use of multiple fuel jets in a single burner. Such burners may employ fuel staging, flue-gas recirculation, or a combination of both. Certain burners may have as many as 8-12 fuel nozzles in a single burner. The large number of fuel nozzles requires the use of very small diameter nozzles. In addition, the fuel nozzles of such burners are generally exposed to the high temperature flue-gas in the firebox.
Because of the interest in recent years to reduce the emission of pollutants and improve the efficiency of burners used in large furnaces and boilers, significant improvements have been made in burner design. One technique for reducing emissions that has become widely accepted in industry is known as staging. With staging, the primary flame zone is deficient in either air (fuel-rich) or fuel (fuel-lean). The balance of the air or fuel is injected into the burner in a secondary flame zone or elsewhere in the combustion chamber. Combustion staging results in reducing peak temperatures in the primary flame zone and has been found to alter combustion speed in a way that reduces NOx. However this must be balanced with the fact that radiant heat transfer decreases with reduced flame temperature, while CO emissions, an indication of incomplete combustion, may actually increase.
In the context of premix burners, the term primary air refers to the air premixed with the fuel; secondary, and in some cases tertiary, air refers to the balance of the air required for proper combustion. In raw gas burners, primary air is the air that is more closely associated with the fuel; secondary and tertiary air is more remotely associated with the fuel. The upper limit of flammability refers to the mixture containing the maximum fuel concentration (fuel-rich) through which a flame can propagate.
U.S. Pat. No. 2,813,578, the contents of which are incorporated by reference in their entirety, proposes a heavy liquid fuel burner, which mixes the fuel with steam for inspiration prior to combustion. The inspirating effect of the fuel and steam draws hot furnace gases into a duct and into the burner block to aid in heating the burner block and the fuel and steam passing through a bore in the block. This arrangement is said to be effective to vaporize liquid fuel and reduce coke deposits on the burner block and also to prevent any dripping of the oil.
U.S. Pat. No. 2,918,117 proposes a heavy liquid fuel burner, which includes a venturi to draw products of combustion into the primary air to heat the incoming air stream to therefore completely vaporize the fuel.
U.S. Pat. No. 4,230,445, the contents of which are incorporated by reference in their entirety, proposes a fluid fuel burner that reduces NOx emissions by supplying a flue gas/air mixture through several passages. Flue gas is drawn from the combustion chamber through the use of a blower.
U.S. Pat. No. 4,575,332, the contents of which are incorporated by reference in their entirety, proposes a burner having both oil and gas burner lances, in which NOx emissions are reduced by discontinuously mixing combustion air into the oil or gas flame to decelerate combustion and lower the temperature of the flame.
U.S. Pat. No. 4,629,413 proposes a low NOx premix burner and discusses the advantages of premix burners and methods to reduce NOx emissions. The premix burner of U.S. Pat. No. 4,629,413 is said to lower NOx emissions by delaying the mixing of secondary air with the flame and allowing some cooled flue gas to recirculate with the secondary air. The contents of U.S. Pat. No. 4,629,413 are incorporated by reference in their entirety.
U.S. Pat. No. 5,092,761 proposes a method and apparatus for reducing NOx emissions from premix burners by recirculating flue gas. Flue gas is drawn from the furnace through recycle ducts by the inspiriting effect of fuel gas and combustion air passing through a venturi portion of a burner tube. Airflow into the primary air chamber is controlled by dampers and, if the dampers are partially closed, the reduction in pressure in the chamber allows flue gas to be drawn from the furnace through the recycle ducts and into the primary air chamber. The flue gas then mixes with combustion air in the primary air chamber prior to combustion to dilute the concentration of oxygen in the combustion air, which lowers flame temperature and thereby reduces NOx emissions. The flue gas recirculating system may be retrofitted into existing burners or may be incorporated in new low NOx burners. The entire contents of U.S. Pat. No. 5,092,761 are incorporated herein by reference.
U.S. Pat. No. 5,516,279 proposes an oxy-fuel burner system for alternately or simultaneously burning gaseous and liquid fuels. Proposed therein is the use of a gaseous fuel jet emanating from an oxy-fuel burner that is either undershot by an oxygen lance or is sandwiched between oxidant jets produced by two subsidiary oxidant jets which are preferably formed of oxygen. An actuable second fuel nozzle is proposed for producing a second fuel jet composed of liquid fuel which is angled toward the oxidant jet at an angle of less than 20°. When liquid fuel is to be used, it is proposed that the gaseous fuel be turned off and the liquid fuel turned on and vice-versa or both can operate simultaneously where the oxidant supplies oxygen to both fuel streams.
U.S. Pat. No. 6,877,980 proposes a burner for use in furnaces, such as in steam cracking. The burner includes a primary air chamber; a burner tube having an upstream end, a downstream end and a venturi intermediate said upstream and downstream ends, said venturi including a throat portion having substantially constant internal cross-sectional dimensions such that the ratio of the length to maximum internal cross-sectional dimension of said throat portion is at least 3, a burner tip mounted on the downstream end of said burner tube adjacent a first opening in the furnace, so that combustion of the fuel takes place downstream of said burner tip and a fuel orifice located adjacent the upstream end of said burner tube, for introducing fuel into said burner tube.
Notwithstanding the widespread use of single fuel burners, there has been considerable interest in dual fuel burners which use both gas and liquid fuels simultaneously. Various benefits can be obtained through the use of a dual fuel implementation. For example, these burners can be designed, in many cases, to permit either dual fuel combustion or gas only combustion and thus provide flexibility in fuel selection. The conventional wisdom when designing dual fuel burners is to supply a large amount of air to the liquid fuel flame in an effort to achieve efficient combustion with minimal carbon and soot production. It is also typical for these burners to have a completely separate gas and liquid flame because it is thought that the gaseous flame has such a high combustion rate that it will use up most of the oxygen and thus deprive the liquid fuel of the oxygen that it needs to provide efficient combustion.
As may be appreciated, one possible fuel for use in a dual fuel burner is steamcracker tar. Steamcracker tar typically has a very low ash content which helps to minimize the amount of particulates ultimately emitted from the flame. However, there are concerns when steamcracker tar is burned in a conventional dual fuel burner particularly in an overly air-rich environment.
First, if too much air is used, the combustion temperature in the burner can become too low. In this event, the combustion efficiency decreases and the carbon production of the burner will increase. Second, flame stability can become an issue in that the flame may oscillate between complete or nearly complete combustion to severely incomplete combustion. As a result of incomplete combustion, a significant amount of soot will be produced by the burner.
Despite these advances in the art, what is needed is a dual fired gaseous/non-gaseous burner that permits flexibility in fuel selection and which has good combustion efficiency, has a stable flame and has low soot production characteristics.